I have experienced this problem for some weeks. Firefox opens certain websites in the regional language, which in my case is German. However, my mother tongue is Spanish and I have my smartphone configured with this language. So, if the website can be displayed in Spanish, Firefox opens it in Spanish. However, if it does not exist in this language, it opens it in German instead of English, which is very annoying. Fortunately, this effect does not occur in the desktop version, so I don't understand why it happens for android. The website where I see this happening is Reddit. I find it impossible to open it in english by default. However, if I use other browsers like the Samsung one, it opens it by default in English. I find this effect so annoying that I'm considering stopping using firefox altogether for Android.

Long story short: If phone language is not supported by the website, firefox opens it in the regional language instead of English.